This invention relates to a modulating relief valve for use on a pressure vessel. Relief valves are commonly used on pressure vessels to prevent pressure inside the vessel from exceeding a limit at which the vessel could explode or otherwise suffer damage.
Presently, many relief valves are of the "snap" variety. See, For example, U.S. Pat. No. 4,291,721. Snap relief valves have only two operating positions: fully open and fully closed. That is, when the pressure inside the vessel reaches the limit, the snap relief valve opens to a fully open position. This causes excessive discharge of fluid and excessive wear and tear on the relief valve.
Some known relief valves are of the "modulating" variety. Modulating relief valves open gradually and proportionally to an amount by which the pressure in the vessel exceeds a predetermined limit. See, for example, U.S. Pat. Nos. 4,586,533, 4,609,008, and 4,917,144. The known modulating relief valves have certain problems associated with them. They lack precision in modulation due to friction and lack of precise negative feedback, and they cannot handle pressures above a certain point. Some known modulating relief valves have multiple pistons with rubber o-rings around those pistons. Those o-rings wear down, causing the rubber to vulcanize inside the relief valve. Some known modulating relief valves have leakage resulting from the moving parts. Also, some known modulating relief valves are difficult and expensive to manufacture.
Therefore, what is needed is a modulating relief valve with improved precision in its modulation that is capable of handling high pressures. Also needed is a type of valve that uses fewer rubber o-rings around moving parts, that has less leakage, and that is easier and less expensive to manufacture.